Why the back wheel of a rear wheel drive can be rotated separately?

Answer 1

For the drive wheels on most cars - the rear wheels on a rear-wheel drive car, or the front wheels on a front-wheel drive car. There is a "limited slip differental", a set of gears connected to the driveshaft and axles. For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another.